1. Field of the Invention
The present invention relates to a secret communication control apparatus which may be used to effect secret communication with communication equipment, for example, facsimile equipment, which transmits picture information by use of public communication lines. More particularly, the present invention pertains to an appendant secret communication control apparatus which is independent of the main body of communication equipment and which enables secret communication by intervening in the communication performed by the communication equipment.
2. Description of the Related Art
A variety of means for encryption and decryption of information for secret communication have heretofore been proposed, and facsimile equipment having a secret communication control function has also been known, for example, Japanese Patent Laid-Open Publication (KOKAI) No. 59-221167 (1984).
The apparatus disclosed in Japanese Patent Laid-Open Publication (KOKAI) No. 59-221167 (1984) has a structure in which a secret communication control funtion is built in facsimile equipment or a secret communication control device is externally connected to facsimile equipment through a predetermined interface. To automatically discriminate between facsimile equipments with and without the secret communication control function, information concerning the secret communication control function is added to the signal NSF (Non-Standard Function) that is defined as an optional function in the communication control procedures recommended by CCITT.
The above-described apparatus, however, involves the problem that, since the secret communication control function discrimination is effected by use of the signal NSF that is not standardized, it is only possible to utilize as remote communication equipments a limited number of equipments that perform the same processing in regard to the secret communication control function discriminating signal, that is, the same type of equipment as the local communication equipment.
Since the secret communication control means is built in or added to the facsimile equipment itself, conventional facsimile equipment cannot be utilized as it is but must be modified or changed in design in order to provide it with the secret communication control means.
Under these circumstances, the present inventor has already invented a secret communication control apparatus designed to be inserted in communication lines (public communication lines) connecting together the facsimile equipment and the exchanger, and filed an application for a patent. Since this secret communication control apparatus performs encryption and decryption of messages for secret communication by intervening between two facsimile equipments, it is possible to effect secret communication by use of conventional facsimile equipments with no secret communication control function. Moreover, since the secret communication control apparatus is inserted in communication lines, it can be completely separated from the main body of the facsimile equipment and can therefore be concealed from a third party.
The conventional secret communication control apparatus of the type described above suffers, however, from the disadvantage that secret communication needs a longer time than ordinay, nonsecret communication.
More specifically, the prior art needs to relay through the secret communication control apparatus all of the various signals and control codes transmitted between two facsimile equipments and consequently, when information is sent from one facsimile equipment to another, it is necessary to effect signal transmission in each of the three communication systems, that is, those between the sending facsimile equipment and the first secret communication control apparatus interposed in the sending line, between the receiving facsimile equipment and the second secret communication control apparatus interposed in the receiving line and between the first and second secret communication control apparatuses. Accordingly, the time required for secret communication is three times that in the case of ordinary communication, even on simple calculation.
Since processing of communication messages, for example, picture data, is relatively simple, encryption and decryption of such data can be executed virtually in a real-time manner and it is therefore possible to simultaneously effect reception and transmission of the data and hence prevent the transmission time from lengthening. However, processing of communication control procedures is so complicated that it is impossible to simultaneously effect reception and transmission of communication control codes, and transmission of communication control codes unavoidably requires a period of time which is three times that in the case of ordinary communication. When a communication error or other trouble occurs, the time required for secret communication further lengthens.